Military and commercial divers are frequently deployed from diving bells and lockout submersibles at very great depths at which the water is frequently very dark and murky. In such a deployment is is essential that the diver should not stray too far above or below the bell or submersible. If he does move out of a very rigidly defined safe depth "window" he is liable to suffer either explosive decompression to the surface (the bends) or "chokes" and suffocation. It is important therefore that the diver should, at all times, have an awareness of the aspect of the true horizon and his relationship to it, i.e. whether he is right side up or upside down, leaning forward or falling backward, etc. Such information is not normally available to the diver as light from above does not reach him and his own physiological sensations are masked because he is usually in a freely buoyant or suspended state.
Heretofore the problem has been overcome by the provision of bright lights suspended from or emanating from the diving bell or submersible, but there remains the not inconsiderable risk that these lights may fail or the water is so murky that the diver cannot see the lights sufficiently clearly for proper orientation. Even if the diver is himself equipped with lights it is often the case that he cannot determine the direction of the horizon or true vertical and he is then in great danger or sinking or rising out of his safe depth or pressure window.